There are numerous reports in the literature on social and reproductive behavior of our closest relatives: the common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus). Data on ovarian activity in these species are less abundant and, in the case of the bonobo, have been based almost exclusively on monitoring the conspicuous perineal swellings of the females. The split between the human line of ancestry and the line of the chimpanzee and bonobo is believed to have occurred a mere six million years ago, while the subsequent divergence of P. troglodytes and P. paniscus lines came much later, possibly prompted by the chimpanzee's need to adapt to relatively open, dry habitats. In contrast, bonobos probably never left the protection of the trees. If this evolutionary scenario of ecological continuity is true, the bonobo may have undergone less transformation than either humans or chimpanzees. It could most closely resemble the common ancestor of all three modern species. Therefore, the proximate, hormonal regulation of sexual swelling and the occurrence of female receptivity and attractivity are central issues for our further understanding of the evolution of hominid socio-sexuality. This project is designed to further the knowledge of reproductive cycles and sexual behavior, and the effects of stress upon them in captive bonobos, employing a multi-institutional approach. It is anticipated that the results will lead to a greater understanding of reproduction of these great apes, and provide substantial baseline information essential for monitoring populations in their natural habitat. Given the close phylogenetic relationship between humans and bonobos, the outcome of this research could well shed light on the mechanisms of sexual dysfunction that may play a major role in today's and future developments of human socio-sexuality. Three questions comprise the scope of the proposed investigation of captive bonobos: I). Are completed copulations accompanied by ejaculations restricted to the most probable time of ovulation as in other great apes, or do they occur throughout the cycle? II). How are development of estrous cycle and adolescent sterility hormonally regulated? III). Does social stress influence highly subordinate females/males in their socio-sexual relationship with dominant males/females?